Field of the Invention
In a melting furnace, the raw materials are converted into molten material by an application of heat energy which is generally supplied at least in part by combustion. Most of the heat energy generated by the combustion is transferred to the charge (solid raw materials and molten material). However, residual energy is removed from the furnace with the combustion fumes.
Related Art
In the case of an aerocombustion, or air-fuelled combustion melting furnace, it is thus known practice to use alternating countercurrent exchangers made up of ceramics (regenerators) or steel exchangers (recuperators) for preheating the combustion air upstream of the furnace so as to increase not only the efficiency of the combustion in the furnace but also the efficiency of the installation overall insofar as some of the heat energy contained in the removed fumes is recuperated and used as energy for preheating the combustion air.
In a glass furnace with recuperators, the combustion air is preheated to 700° C. whereas regenerators allow combustion air temperatures of 1200° C. or even 1250° C. to be achieved at the start of life of the installation.
The operators of melting furnaces, particularly glassmakers, are increasingly adopting oxycombustion technology, which is both more effective (because it eliminates the thermal ballast of the nitrogen) and less polluting (reducing the NOx and CO2, as it is this same nitrogen that is the origin from which the NOx is formed).
However, the systems for recuperating energy from fumes that have been developed for aerocombustion (regenerators and recuperators) are not well suited to the recuperation of heat energy from the fumes generated by oxycombustion.
EP-A-1338848 describes a system for recuperating energy from the fumes of a glass furnace, particularly an oxycombustion glass furnace. Said system comprises at least one heat exchanger for preheating an oxygen-rich gas and/or a gaseous fuel by exchange of heat with the fumes removed from the furnace, a boiler situated downstream of the at least one heat exchanger and able to generate superheated steam by exchange of heat with the fumes and a steam turbine for expanding the superheated steam to produce mechanical energy.
According to EP-A-1338848, the mechanical energy generated by the turbine can be used to fulfill at least some of the energy requirements of an installation for separating the gases of the air which supplies combustion oxygen for the glass furnace.
In order to produce superheated steam in the boiler at an industrially acceptable efficiency, the fumes at the inlet to the boiler, and therefore also at the outlet of the heat exchanger, need to be at a temperature of at least 1000° C., or even of 1200° C. to 1500° C.
Despite the good ability of the materials identified in EP-A-1338848 to withstand such temperatures, glassmakers prefer to use lower-temperature energy recuperation systems which are considered to be more durable.
Such an alternative system that is particularly reliable at recuperating energy from the fumes of an oxycombustion glass furnace is described in EP-A-0872690.
According to EP-A-0872690, the fumes originating from the oxycombustion furnace are used for the indirect preheating of the oxygen and/or of the fuel upstream of the furnace. In a first heat exchanger, the fumes from the furnace heat an intermediate fluid, such as air for example, by exchange of heat between the two fluids. The heated intermediate fluid from the first exchanger is used in a second heat exchanger to heat combustion oxygen and/or the fuel.
The system for recuperating energy from fumes according to EP-A-0872690 does not, however, allow additional recuperation of energy from the fumes in the form of superheated steam, as is the case in EP-A-1338848, because in practice the fumes at the outlet of the first exchanger are at a temperature markedly below 1000° C.